It used to be 50, I think ~9 gives best results for Chromium on Win:
  on golo VM, it takes <4 minutes to re-boostrap + git fetch small
  delta, assuming zipped git checkout for bootstrap is fresh (~1day).

For other repos, which are significantly smaller, this change should
have minor effect if at all.

Test: I tested this using `led` tool on Win7 machines running LUCI
stack extensively. For example,

  * https://ci.chromium.org/swarming/task/3a0102e8c8657410
    shows case with few .pack files, hence just 1 fetch

  * https://ci.chromium.org/swarming/task/3a010282f9fd8010
    shows case with 39 .pack files and so bootstrapping + fetch.
    If you look at prior tasks on the same VM, you'd find this:
    https://ci.chromium.org/swarming/task/39ffe843d01ed010
    which spent 8 minutes doing 1 incremental fetch with 39 .pack
    files.

**Troopers/Sheriffs**: This change is safe to revert.
However, beware that you should also at the same time revert the recipe
roll of this CL to the repo, in which the failed builder's recipe is
located, typically `chromium/tools/build`.

Bug: 749709
Change-Id: I18e2b63283100d466e9fb981a9094862463f6909
Reviewed-on: https://chromium-review.googlesource.com/787174
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@google.com>